Currently, ProcessMaker does not support the option to send emails using a text plain format because you need a template to send an email message and a template uses the HTML format. Nevertheless, you can use the notification task property to send an email as a plain text.

Unfortunately, there is no way to do it from inside the PM interface. However, you can edit the template file which is stored at /shared/sites/<WORKSPACE>/mailTemplates/<PROCESS-UID>/<FILENAME>.html on the PM server.
You can get the workspace and unique ID of the process, by running a case with Debug Mode enabled, and looking at the SYS_SYS and PROCESS variables.

Remove the HTML formatting in the file and it should just send plain text. Don't change the filename and leave the extension as .html . If you change the filename, you will have to also change it in the database.